In the semiconductor manufacturing industry, it is necessary to achieve precise control of the quantity, temperature and pressure of one or more reactant materials which are delivered in a gaseous state to a reaction chamber. Some process reactants, such as nitrogen gas, are relatively easy to deliver in a controlled manner at the temperatures and pressures required for the reaction to occur. Other reactants, however, may be highly corrosive, toxic, pyrophoric, or unstable at the temperatures and/or pressures at which delivery to the reaction chamber is required. Such characteristics of the reactants make their accurate and controlled delivery to a reaction chamber extremely difficult to achieve.
Mass flow controllers (hereinafter, “MFCs”) are widely used in the industry to control the delivery of process reactants. Two broad categories of MFCs, thermal and pressure-based, have been developed to handle the diverse delivery requirements of a wide variety of process reactants. A mass flow controller generally includes a mass flow measurement apparatus for measuring the rate of flow of gas through the controller, a valve for controlling the flow of gas through the controller and a computer connected to the mass flow measurement apparatus and the valve. The computer is programmed with a desired flow rate, which the computer compares to an actual flow rate as measured by the mass flow measurement apparatus. If the actual flow rate does not equal the desired flow rate, the computer is further programmed to open or close the valve until the actual flow rate equals the desired flow rate.
Thermal mass flow controllers operate on the principle that the rate of heat transfer from the walls of a flow channel to a fluid flowing in laminar flow within the channel is a function of the difference in temperatures of the fluid and the channel walls, the specific heat of the fluid, and the mass flow rate of the fluid. Thus, the rate of mass flow of a fluid (in the laminar flow regime) can be determined if the properties of the fluid and the temperatures of the fluid and tube are known.
On the other hand, pressure-based MFCs establish a viscous flow condition by creating two pressure reservoirs along the flow path of a fluid, for example, by introducing a restriction in the diameter of the flow path. The restriction may comprise an orifice or nozzle. In the reservoir upstream of the flow restrictive aperture, the fluid has a pressure P1 and a density ρ1, which can be used to determine the flow with a known aperture under viscous chock flow conditions.
It is also known to measure gas flow rates with a hot wire anemometer. In a hot wire anemometer, the gas typically is passed over a single heated wire, reducing the temperature of the wire. The change in resistance of the heated wire is determined and correlated with the flow rate of the gas. A more advanced technique employs a second heated wire positioned downstream of the first heated wire. The gas is passed through the system, reducing the temperature of the upstream wire and increasing the temperature of the downstream wire. The temperature difference is then recorded as an output signal.
What is still desired, however, is a mass flow controller including a new and improve apparatus and method for measuring rates of mass flow. Preferably, the new and improved apparatus and method will utilize a hot wire anemometer to measure rates of mass flow. In addition, the new and improved apparatus and method will preferably be material compatible with the gas being measured, not be adversely effected by vortex shedding, be insensitive to gas type, and insensitive to ambient temperature changes. Moreover, the new and improved apparatus and method will preferably respond quickly to changes in flow rates and will be able to measure a wide range of flow rates.